1. Field of the Invention
This invention relates to carrier complexes for use in apparatus and methods for extracting small ligands from a fluid; more particularly, the invention relates to electrochemically active macrocyclic polyamine (also referred to as macrocyclic amine) complexes of transition metals that reversibly bind small ligands such as molecular oxygen and the use of such complexes for extraction of ligands from a first fluid environment and release ligands to a second fluid environment.
Purified gases, such as oxygen, are useful in a number of industrial, scientific and medical applications. Such gases may be obtained in a variety of ways. Large-scale extraction of gases from air may be accomplished by cryogenic fractionalization where the air is liquified and separated based on the differing boiling points of its constituent gases. Although practical for producing large volumes of gases, cryogenic fractional distillation is impractical for supplying small gas volumes, particularly at remote or inaccessible locations.
As an alternative to cryogenic fractionalization, a variety of small-scale techniques have been developed for producing relatively pure gases. For example, hydrogen and oxygen may be produced by the electrolysis of water under controlled conditions. Although practical for many applications, electrolysis suffers from relatively high energy requirements and a substantial danger of explosion resulting from the presence of molecular hydrogen. Methods have also been developed for extracting dissolved gases from liquids, particularly fresh sea water. Such methods generally employ gas-permeable membranes for extracting the gases. As with electrolysis, membrane gas extraction is useful, but suffers from a number of limitations. In particular, most membranes are nonselective and will pass whatever gases are dissolved in the sea water. Moreover, the pressure of the collected gas generally cannot exceed the partial pressure in the sea water, at least in the absence of suitable compression and storage equipment.
Recently, systems have been developed for extracting oxygen from fluid mixture feedstocks based on the use of organometallic carrier compounds which in a first oxidation state bind the oxygen molecules and in a second oxidation state release the oxygen molecules. The systems, as described in U.S. Pat. Nos. 4,602,987, 4,609,383, and 4,629,544 rely on circulating the carrier compounds past a first location where the oxygen is bound, typically through an oxygen-permeable membrane. The oxygen-loaded carrier compounds are circulated past a first electrode where their oxidation state is changed, causing release of the oxygen which then may be collected and stored or utilized. The unloaded carrier compounds are then circulated past the second electrode of the electrochemical cell, where they are returned to their first oxidation state. The carrier compounds are then returned to the loading station where they can again bind oxygen from the fluid mixture.
Such systems have several advantages. First, the energy requirement is low relative to other extraction techniques, particularly electrolytic decomposition of water. Second, the partial pressure of oxygen which may be obtained is limited only by the solubility of the carrier complexes in the circulating carrier fluid. Thus, oxygen pressures which are much higher than the partial pressure in the fluid mixture may be obtained without use of supplemental compression equipment.
Despite the substantial advances represented by U.S. Pat. Nos. 4,602,987, 4,609,383, and 4,629,544, it would still be desirable to provide improvements in the systems described. For example, the efficiency of oxygen extraction systems could be increased by optimizing the structures, and hence the electrochemical, thermodynamic, and kinetic properties of the carrier compounds. It would also be desirable to provide a more efficient transfer of electrons from the carrier compounds to the anodic electrode and from the cathodic electrode to carrier compounds. Further, enhanced oxygen extraction by promoting the transfer of electrons from the carrier compounds to the anodic electrode and/or from the cathodic electrode to the carrier compounds would be possible. Such increased efficiencies would increase the volume output of oxygen from a fixed sized cell or, alternatively, allow a fixed amount of oxygen to be produced by a cell having reduced electrode area and/or lower power consumption.
The use of transition metal complexes of linear, pentadentate polyalkylamines in electrochemical oxygen extraction and generation processes is described in copending applications, assigned to the same assignee as the present application: Ser. No. 018,891, filed Feb. 25, 1987; Ser. No. 018,895, filed Feb. 25, 1987; and Ser. No. 018,888, also filed Feb. 25, 1987; the disclosures of each of these copending applications are hereby incorporated herein by reference.
2. Description of the Background Art
Some types of such transition metal carrier complexes have been used in or suggested for use in devices for extraction, absorption, and generation of oxygen from fluid media. For example, Roman, U.S. Pat. Nos. 4,451,270 and 4,542,010, discloses various metal complexes in a non-electrochemical oxygen extraction system utilizing an oxygen selective, permeable membrane. The carriers include cobalt complexes of linear and macrocyclic tetradentate, linear pentadentate, and bindentate Schiff base ligands in primarily non-aqueous, Lewis base solvents; all disclosed systems appear to require the use of an axially-coordinating base. Hill, U.S. Pat. No. 4,442,297, uses phosphine complexes of Mn(II) in dehydrated solvents to purify nitrogen gas by extracting impurities including molecular oxygen. Sievers, U.S. Pat. No. 4,514,522, discloses oxygen sorbents comprising linear, tetradentate ketoamine complexes bound to porous polymers. Gagne, U.S. Pat. No. 4,475,994, uses cobalt complexes of unknown stoichiometry in a mixed solvent at high pH to transport electrochemically generated superoxide ions across a fluid membrane. Bonaventura, et al., U.S. Pat. Nos. 4,602,987; 4,609,383 and 4,629,544, disclose a variety of metalloporphyrins and macrocylic ligand complexes, in combination with axially coordinating Lewis bases, in aqueous, non-aqueous, and water-immiscible solvents and their use to electrochemically separate oxygen from fluids.
Oxygen carrier compounds, including cobalt complexes of some macrocyclic amines, and their thermodynamic properties have been extensively reviewed and tabulated. Niederhoffer, et al., Chem. Rev. 84 137-203 (1984).
Oxygenation equilibria and kinetics of macrocyclic amine complexes of cobalt ion have been studied by C-L. Wong, et al., 102 J. Am. Chem. Soc. 5511-18 (1980). T. Geiger and F. C. Anson, 102 J. Am. Chem. Soc. 7489-96 (1981) have investigated the catalysis, by Cobalt (III) complexes of macrocyclic amines, of the electromechanical reduction of molecular oxygen. The use of such complexes for the electrochemical extraction and regeneration of oxygen is not known.